As a society, we routinely rely upon a broad class of devices known as vacuum electronics to help us communicate, understand weather, maintain air safety, image and diagnose medical conditions, sustain our national defense, and for other applications. These important devices utilize an electron beam to amplify or create radiation, and include the microwave tubes found in satellite and ground communications, x-ray tubes in medical imaging and airport screening systems, civilian and military microwave systems, cellular network nodes, etc. The technology has become central to defense and military systems, but those of skill in the art also recognize limitations which erode their ability to meet future needs. These limitations are directly tied to the method by which the requisite electron beams are created, which is now more than 50 years old.
Currently, two practical options exist to provide electron sources for the vacuum electronics industry: thermionic cathodes and photocathodes. Increasingly, though, new high-power, high-frequency (>100 GHz) devices are limited by the properties of these sources. Of particular concern are beam quality, lifetime and ruggedness.
Thermionic cathodes are typically made of pure tungsten, or barium or strontium oxides impregnated in a matrix of porous tungsten. They are a well known and reliable technology that has been used over many decades. However, they are limited to low current densities (<10 A/cm2), requiring large cathode areas for high average or peak beam currents. Furthermore, they must be heated to high temperature (1400 K to 2500 K), which requires extra power and makes them susceptible to damage in poor vacuum environments, thus exacerbating emittance concerns. The high thermal gradients between the cathode and adjacent device components introduce expensive engineering challenges and results in undesirable transverse beam energies of 0.1 eV or greater. The combination of large cathode area and the transverse energy typically result in relatively low quality electron beams.
Photocathodes are a more recent development, able to source high current densities (100s of A/cm2) with prompt emission, effectively imitating the shape of the laser pulse used to drive them. However, they require a sophisticated laser system, are limited to low average current, and typically emit thermally “hot” electrons (i.e., having about 1 eV transverse temperature) due to the difference between the laser photon energy and the work function of the photocathode material.
An ongoing need exists, therefore, for an electron beam source suitable for use with vacuum electronics which produces high beam quality, has good lifetime, and is low-maintenance.